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Abstract

Here, we demonstrate the simple fabrication of a single-walled carbon nanotube (SWCNT)
field emission electrode which shows excellent field emission characteristics and
remarkable field emission stability without requiring posttreatment. Chemically functionalized
SWCNTs were chemically attached to a silicon substrate. The chemical attachment led
to vertical alignment of SWCNTs on the surface. Field emission sweeps and Fowler-Nordheim
plots showed that the Si-SWCNT electrodes field emit with a low turn-on electric field
of 1.5 V μm−1 and high electric field enhancement factor of 3,965. The Si-SWCNT electrodes were
shown to maintain a current density of >740 μA cm−2 for 15 h with negligible change in applied voltage. The results indicate that adhesion
strength between the SWCNTs and substrate is a much greater factor in field emission
stability than previously reported.

Keywords:

Background

Carbon nanotubes (CNTs) have a high aspect ratio and high electrical conductivity
which make them prime candidates for field emission electrodes [1,2]. The practical application of a CNT-based field emission device requires both a low
turn-on electric field (Eto) and a stable output current [3]. Single-walled carbon nanotubes (SWCNTs) are accepted to have excellent field emission
properties including a low turn-on field and high electric field enhancement factor
(β) since their small diameter provides the highest aspect ratio compared with multi-walled
carbon nanotubes (MWCNTs) [4-6]. Conversely, field emission from SWCNTs is usually regarded to be fragile because
the single-shelled SWCNTs are less resilient to emission degradation mechanisms such
as ion bombardment and Joule heating. Recently, much work has focused upon improving
the field emission properties of MWCNTs due to their inherent emission stability [7,8]. Posttreatments to SWCNT films, such as plasma exposure, have been shown to significantly
increase emission stability but at the cost of increasing Eto[9]. Improving the emission stability from SWCNT electrodes without adversely affecting
Eto and β is an ultimate goal in the field [3].

We have recently reported field emission from SWCNTs chemically attached to silicon
and showed that these devices could withstand field emission current densities up
to 500 μA cm−2 and were relatively stable, with the voltage required to maintain a current density
of 95 μA cm−2 only increasing by 15% after 15 h and by 36% after 65 h [10]. More recently, we investigated field emission properties and stability from functionalized
single-, double-, and multi-walled CNTs chemically attached to silicon where we found
that the degree of functionalization played a major role in emission stability [11].

Recent experiments on Si-SWCNT electrodes have shown that a 2-h attachment time yields
superior photovoltaic and electrochemical devices [12-14]. The field emission stability of 2-h Si-SWCNT electrodes has not been previously
investigated. In this letter, we improve significantly upon previous Si-SWCNT electrodes
and demonstrate that the chemical attachment of SWCNTs to a silicon substrate is a
simple route toward the fabrication of a SWCNT electrode with stable emission while
maintaining excellent values for Eto and β.

Methods

Si-SWCNT electrodes were fabricated following the chemical attachment of functionalized
SWCNTs as described in detail elsewhere [15]. Briefly, n-type highly antimony-doped Si wafers were cut to 0.25 cm2 and cleaned ultrasonically in acetone for 2 min. The Si wafers were then hydroxylated
by stepwise immersion in 1:1:5 NH4OH:H2O2:H2O followed by HCl:H2O2:H2O for 20 min at 80°C. The wafers were then incubated for 2 h in a solution of dimethyl
sulfoxide containing 0.2 mg mL−1 carboxylic acid-functionalized SWCNTs and dicyclohexylcarbodiimide [15]. The Si-SWCNT wafers were then washed ultrasonically for 2 min and dried in a stream
of nitrogen. Field emission measurements were collected using a parallel plate setup
with Si-SWCNT electrodes as the cathode and a stainless steel disk as the anode separated
by 1.82 mm as determined by a micrometer screw [11]. All measurements were taken using a LabVIEW-controlled Keithley source measure unit
(Keithley Instruments Inc., Cleveland, OH, USA). The base pressure of the field emission
testing system was <1 × 10−8 Torr.

Results and discussion

Figure1 presents an atomic force microscopy (AFM) image of the prepared Si-SWCNT electrode.
The SWCNTs form vertically aligned bundles, consistent with previous observations
for SWCNTs chemically attached to substrates using this method [16-18]. Vertical alignment of the SWCNTs is due to two main effects: (a) the hydrophilic
hydroxyl groups on the surface repel the hydrophobic walls of the SWCNTs, forcing
the SWCNTs away from the surface; and (b) the high density of carboxylic acid groups
on the ends of the SWCNTs promotes end-on surface attachment. The SWCNT bundle density
is in the order of approximately 1 × 109 cm−2 with heights of the features observed ranging from 20 to 100 nm with an average bundle
diameter of 75 nm as determined by AFM analysis.

A field emission JV sweep and its corresponding Fowler-Nordheim (F-N) plot for the Si-SWCNT electrode
is shown in Figure2. The maximum field emission current density (Jmax) measured was 790 μA cm−2 and was limited by the maximum current that could be supplied by the field emission
apparatus, indicating that Jmax was actually higher than 790 μA cm−2. Field emission is demonstrated by the highly linear F-N plot [19], and from the slope of this plot, β was calculated to be 3,965 (assuming a SWCNT work function of 4.8 eV) [20]. In addition, these devices exhibited an Eto (electric field for J = 10 μA cm−2) and an Eth (electric field for J = 100 μA cm−2) of 1.5 V μm−1 and 2.37 V μm−1, respectively.

The low Eto and high value for β demonstrate that the Si-SWCNT electrode is an elite field emission device [3,6,11,21,22]. The measured values are perhaps surprising given the short length and wide bundle
diameter of the SWCNTs as determined by AFM. We hypothesize that by using a solution
containing SWCNTs with a variety of lengths, we have produced a surface whereby the
electrical field screening between SWNTs is minimal, leading to the excellent field
emission characteristics observed [23]. Indeed, this hypothesis is supported by Chhowalla et al. who showed that short and stubby CNTs outperformed taller and thinner CNTs [24]. They argued that electrical field screening between adjacent CNTs was reduced when
the CNT forest did not have a uniform height and the CNTs had a greater spacing.

The emission stability from the Si-SWCNT electrode was tested by monitoring the voltage
required to maintain a current of 740 μA cm−2 for 15 h. The variation of both the applied voltage (V-t) and the current density (J-t) as a function of time are presented in Figure3. There appears to be three main regions to the V-t and J-t plots:

1. For t < 0.5 h, the field emission is relatively unstable with both V and J fluctuating as a function of time, which is consistent with previous observations
[25] and is most probably related to field-induced desorption of adsorbates, causing fluctuations
in both the work function and β[26-28].

2. For 0.5 < t < 2.5 h, V is essentially constant while J increases to 770 μA cm−2 at t = 2.5 h. This observation is consistent with a decrease in sample resistance due
to the removal of amorphous carbon or non-emitting SWCNTs, resulting in improved field
emission [10].

3. For 2.5 < t < 15, J remains constant while V increases slowly from 5,500 V to approximately 5,700 V at t = 15 h, which is consistent with slow degradation of the field emission properties
of the SWCNT film most likely due to ion bombardment and Joule heating processes [27-30].

Figure 3.Field emission stability of Si-SWCNT electrode. Variation of both applied voltage (V-t) and current density (J-t) recorded with respect to time. Dashed red lines were added to signify the three
regions of emission stability discussed in the text.

A total voltage change of 1.8% over 15 h for an applied current of approximately 750
μA cm−2 is very low for a SWCNT field-emitting electrode. The only SWCNT electrode in the
literature with greater stability was a screen-printed SWCNT electrode that was treated
with an Xe/Ne plasma to improve stability to the point where a current density of
100 μA cm−2 was maintained for >50 h with minimal degradation [9]. However, the plasma treatment increased the Eto of the electrode from 2.9 V μm−1 to 4.3 V μm−1, effectively negating the advantages of using SWCNTs. Moreover, the output current
density described here is over seven times greater with negligible field emission
degradation observed.

The highly stable electron field emission at a relatively high J that is observed for the Si-SWCNT electrodes reported in this letter is attributed
to a number of factors. First, the strong chemical attachment between the SWCNTs and
the substrate will reduce the occurrence of field-induced CNT desorption, resulting
in a more consistent field emission. Second, the bundling of the SWCNTs on the surface
may also assist with the observed low degradation, with the outermost SWCNTs protecting
the inner SWCNTs from ion bombardment. Third, as we have previously shown, the high
crystallinity of these SWCNTs improves emission stability [11]. Finally, we propose that variation in the length of the emitting structures results
in low electric field screening of the surface and a concomitant large population
of emitting SWCNTs, leading to a high field emission current density [23].

Conclusions

In summary, a field-emitting electrode consisting of SWCNTs chemically attached to
a silicon substrate has been produced. The Si-SWCNT electrode was shown to field emit
with an Eto of 1.5 V μm−1 and β of 3,965. The emission was shown to be remarkably stable with a current of approximately
750 μA cm−2 maintained for 15 h with a net voltage increase of only 1.8%. The chemical attachment
of SWCNTs to Si is a simple, upscalable approach to produce SWCNT field emission electron
sources with excellent characteristics and stability without the need for posttreatment.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

CJS prepared and characterized the Si-SWCNT electrodes and completed the field emission
experiments with the assistance of AF, MB, and PCD using an apparatus maintained by
AF, MB, and PCD. The experiments were conceived by JS, PCD, and CJS. All authors read
and approved the final manuscript.

Acknowledgments

This work was funded by Flinders University, Westfälische Wilhelms-Universität Münster,
University of Newcastle and the Australian Research Council.